STRUCTURAL CHARACTERIZATION OF SOL-GEL DERIVED OXYCARBIDE GLASSES .1. STUDY OF THE PYROLYSIS PROCESS

A gel resulting from the cohydrolysis of dimethyldiethoxysilane, a siloxane precursor, and tetraethoxysilane, a silica precursor, has been pyrolyzed to a silicon oxycarbide network. This conversion has been carefully characterized using various structural techniques: thermogravimetric analysis coupled with mass spectrometry, multinuclear solid-state magic angle spinning nuclear magnetic resonance (Si-29, H-1, and C-13), infrared spectroscopy, and electronic spin resonance. The pyrolysis process can be divided in two main stages. In the 400-650-degrees-C temperature range, redistribution reactions between Si-O and Si-C bonds occur with evolution of low molecular weight siloxanes. New sites are created, but the methyl groups remain intact. From 650 to 1000-degrees-C, the conversion of the polymeric network into an inorganic material takes place. Cleavage of C-H, Si-C, but also Si-O bonds occurs with evolution of methane, hydrogen and water. Aromatic carbon units are formed. Heat treatment at 1000-degrees-C in argon flow leads to the formation of an amorphous silicon oxycarbide phase, characterized by SiCxO4-x units. The presence of a free carbon phase has also been shown.